As there has been a trend in marketing video cameras to shift from an image size of about 1/3 inch in height to a smaller image of about 1/4 inch in height, there has also been a demand for simplification of the zoom lens for such video cameras. A zoom lens of the rear-focus type is known for use in video cameras wherein the lens consists of four lens groups and power is varied by shifting the second lens group from the object side along the optical axis so as to vary the focal length of the lens. The first lens group and third lens group from the object side remain at a fixed location. A fourth lens group from the object side is shifted so as to maintain the image surface fixed during zooming and focusing on objects at different object distances. Examples of such a rear-focus zoom lens are disclosed in, for instance, Japanese Patent Bulletin No. 2740890 and Japanese Laid-open Patent Application No. H7-199071.
In the zoom lens disclosed in the above-noted Patent Bulletin, predetermined conditions are satisfied regarding: 1) the image magnification ratio between the telephoto end and the wide-angle end when the lens is focused at infinity, 2) the focal length ratio between the telephoto end and the wide-angle end, and 3) the lateral magnifications of the third lens group and the fourth lens group at the telephoto end and the wide-angle end when the lens is focused on a nearby object. The lens has a simple configuration and the fluctuation of variable power is small when the object distance is varied.
The zoom lens disclosed in the above-noted Japanese Laid-open Patent Application has a configuration so as to obtain: 1) predetermined values of refractive power of the second lens group and the fourth lens group, 2) a predetermined refractive power ratio between the second lens group and the fourth lens group, 3) a predetermined refractive power ratio between the first lens group and the third lens group, and 4) a predetermined refractive power of the third lens group so that the lens is compact and aberrations are favorably corrected.
Moreover, in a rear-focus zoom lens consisting of four lens groups as described above, image locations of the first lens group differ depending on the object distance, and the second lens group is essentially immobile in the case where the object is at a maximum near distance or at a maximum telephoto distance. Thus, there is a problem in that the magnification of the lens varies depending on object distance. In particular, the magnification decreases at very near object distances.
In order to prevent such a decrease in magnification, the fourth lens group position can be made to compensate by varying its range of movement, but as a result the overall length of the lens will increase. In other words, as a zoom lens with a larger mobility of the fourth lens group is installed in a camera, the camera will become larger and thus less compact.
On the other hand, when a lens is made compact by increasing the refractive power of each lens group and by reducing the mobility of the fourth lens group, refractive power of the lens concentrates. As a result, the fluctuation in lens properties becomes larger. Thus, it will become difficult to provide high picture quality for all object distances over the entire range of zoom.
In the above-noted rear-focus zoom lens described in Patent Bulletin No. 2740890, the third lens group consists of as many as four or five lens elements. Thus, such a design does not fully accomplish the objective of making the lens compact while providing a high picture quality.